To account for species differences in social organization of wintering bird populations I propose a study of the movements and interactions of individuals composing the social group, the spatial and temporal distribution of resources, and the genetic correlates of dominance rank and survival. The intention is to produce an evolutionary explanation of relatively simple social organizations by focusing on the selective forces that result in differential mortality of phenotypes. Primary data will come from field observations and from laboratory and field experiments on wintering populations of several avian species in a rigorous environment. Preliminary evidence suggests a working hypothesis that age, degree of genetic heterozygosity, and dominance rank in a flock are highly correlated. Social organizations of species forming large flocks (Slate-colored Junco, Tree Sparrow), small flocks (Tufted Titmouse, Black-capped Chickadee), and a species that is solitary and territorial all year (White-breasted Nuthatch) provide the variation that is the backbone of a comparative evolutionary study.